Through-hole drilling is a basic process carried out in the microelectronics manufacturing industry. Exemplary target materials in which through-holes are typically drilled include multi-chip modules (MCMs), ball grid arrays, pin grid arrays, circuit boards, glass cloths (including FR4), hybrid microcircuits, and semiconductor microcircuits. These target materials typically include separate component layers of metal, organic dielectric, and/or reinforcement materials. Through-holes are typically drilled mechanically; however, the growing need for small diameter (e.g., less than 150 microns) through-holes has made mechanical drilling of them impracticable. As the need for small-diameter through-holes increases, there is a consequent increased need for high aspect ratio through-holes (through-holes for which the hole diameter to hole depth ratio is equal to or less than 1.0). High-power laser drilling has been used almost exclusively to form these through-holes.
Exemplary high-power lasers that are used to drill through-holes include ultraviolet (UV) lasers, carbon dioxide (CO2) lasers, and excimer lasers. Drilling involves directing the laser to impinge the target material at a desired spot and ablating the entire thickness of the target material to form a through-hole at the desired spot. During drilling, ablated material is removed through the top opening of the hole. Once the through-hole is completely formed, the remaining ablated material may also be removed through the bottom opening of the through-hole. Although laser drilling facilitates the formation of small-diameter through-holes, through-hole laser drilling remains an imperfect method plagued by various problems.
One problem with laser-drilling through-holes in nonhomogeneous substrates is that they typically exhibit poor wall quality. As the high-power laser drills deeper into the target material, more of the thermal energy added remains in the hole than can escape through the top opening, resulting in a significant amount of thermal energy diffusing through the hole wall into the target material matrix. This diffusion causes significant structural damage to the target material located within the heat affected zone. Further, the inability to efficiently remove drilled target material through the top opening of the through-hole results in the formation of through-holes having undesirable fiber protrusions and/or resin etchback.
A second problem with laser-drilling through-holes is the formation of grooves on the through-hole walls, which result from interaction between the laser beam and the target material. For example, use of a CO2 laser to drill ferrous materials results in the following cyclical process: an exothermic reaction occurs in the spot area, causing rapid expansion of the molten material; once the reaction front leaves the spot area, the molten material resolidifies. This entire cycle is repeated at various depths, creating a periodic grooved structure depthwise along the cut surface.
A third problem with laser-drilling through-holes is that a portion of the drilled target material can partly block subsequent laser pulses or form a plasma plume. For example, target material debris remaining in the hole during laser drilling often ignites and etches out into the heat affected zone. Because a gas plume typically ignites within 10 nanoseconds of the start of each laser pulse, approximately 80% of each laser pulse includes a plasma plume. This occurrence typically results in undesirable resin etchback.
Additionally, superheated debris can change the nature of the ablation process from predominantly ablative to less ablative and more thermal, thereby creating a saturation depth that unfavorably imposes a practical limit on the power density and repetition rate of the high-power laser. Limiting the power density and repetition rate limits the extent to which the ablation rate can be increased.
A fourth problem with laser-drilling through-holes occurs when forming through-holes in highly heterogeneous materials, such as FR4 cloths, which are organic polymer resin-impregnated glass cloths sandwiched between electrically and thermally conductive metal layers. Because the laser ablation characteristics, such as melting and vaporization temperatures, differ for each individual layer, laser processing through-holes is very difficult. For example, the vaporization temperatures of woven glass reinforcement and polymer resin matrix differ greatly. Silicon dioxide (a commonly used woven glass reinforcement) has a melting temperature of about 1,970 Kelvin (K) and a vaporization temperature of about 2,503K. In contrast, organic resins (i.e., epoxies) vaporize between about 500K and about 700K. The disparity in vaporization temperatures makes it difficult to laser-ablate the glass component without ablating the resin surrounding individual glass fibers or in regions directly adjacent to fiber bundles.
Thus far the primary industrial solution to minimizing the detrimental effects consequent to forming through-holes with high-power lasers is to optimize the cutting parameters and, in particular, to use defined laser pulses. However, the degree of improvement that can be made using this method is relatively minimal because optimization of one parameter is often detrimental to a separate aspect of through-hole formation.
Although high-power laser drilling is an integral part of semiconductor micromachining technology, no economically efficient laser drilling method has been found to drill through-holes having smooth walls with a reduced incidence of cut-surface groove formation, fiber protrusion, or fiber etchback. This invention addresses the desire to use a high-power laser to drill through-holes having smooth walls.